Optical interferometers have found application as all-optical switches. In one such device an optical signal at a first wavelength is coupled to an input port of the interferometer which signal is split into two signal portions which later combine to provide an output signal at an output port. The intensity of the output signal is dependant on the relative phase difference between the portions when they combine. A control optical signal is coupled to the interferometer such that the phase difference between the signal portions at the output port, and hence the intensity of the output signal, is dependant on the intensity of the control optical signal.
An example of such optical switching is described in the applicant's co-pending patent application GB8920913.4. The interferometer is configured as a Sagnac loop fabricated from an optical fibre which exhibits a non-linearity, ie. its refractive index varies with the intensity of the optical signal. A cw optical source provides a cw optical signal at the first wavelength which is coupled to the input port which is split into two counter-propagating portions. A control signal at a second wavelength, in this case a received optical data signal, is coupled to the loop so that it propagates along the fibre loop in substantially one direction only.
For a zero intensity data optical signal the first optical fibre coupler and the optical fibre, which form a Sagnac antiresonant interferometer, act as a fibre loop mirror to the cw probe optical signal in that the signal entering the coupler at a first port will be reflected, i.e. it will exit from that same port. This is because the two counter-propagating portions maintain the same relative phase. When an additional optical signal is propagated along a non-linear interaction section of the loop, a phase shift is induced in that first portion of the optical signal which co-propagates with it, the condition for reflection is broken and some of the probe optical signal will exit the second port.
Other configurations of interferometer may be used, for example Mach-Zehnder interferometers. The Sagnac loop has the advantage that the counter-propagating signals transverse the same fibre but has the disadvantage that it is a two-port device. This disadvantage can be overcome by using the four-port Mach-Zehnder, for example.
Other forms of loop interferometer are known, for example waveguide loops fabricated in planar substrates. These can also be used to for non-linear interferometers for use as optical switches.